The MRAM is a promising non-volatile memory that achieves high-speed write operation and an increased number of rewrite operations. A typical MRAM includes a memory cell array within which a plurality of magnetoresistance elements, which function as memory cells, are arranged in rows and columns. The magnetoresistance element is composed of a fixed ferromagnetic layer having a fixed spontaneous magnetization, and a free ferromagnetic layer having a reversible spontaneous magnetization (simply referred to as magnetization, hereinafter), and a spacer layer disposed between the fixed and free ferromagnetic layers. The free ferromagnetic layer is formed to allow the magnetization to be directed parallel or antiparallel to the magnetization of the fixed ferromagnetic layer.
Such designed magnetoresistance element stores one-bit data as the relative direction of the magnetizations of the fixed and free ferromagnetic layers. The magnetoresistance element is allowed to be placed into a “parallel” state in which the magnetizations of the fixed and free ferromagnetic layers are parallel, or an “antiparallel” state in which the magnetizations of the fixed and free ferromagnetic layers are parallel are antiparallel. The magnetoresistance element stores one-bit data, allocating one of the parallel and antiparallel states to logic “0”, and the other to logic “1”.
Write operation into a magnetoresistance element is achieved through developing a magnetic field by flowing a write current through a wiring disposed near the magnetoresistance element, and redirecting the magnetization of the free ferromagnetic layer with the developed magnetic field. The direction of the current is selected in accordance with the desired direction of the magnetization of the free ferromagnetic layer.
In order to reduce the consumption current of a MRAM, it is desired to reduce the current to reverse the magnetization of the free ferromagnetic layer (that is, the write current). One technique for reducing the write current is to provide a yoke layer of ferromagnetic material around the wiring through which the write current is developed. The yoke concentrates the magnetic field on the MRAM memory cell, and thereby effectively reduces the write current. Yoke-including MRAMs are disclosed in Japanese Laid Open Patent Application No. Jp-A 2002-110938, U.S. Pat. No. 6,211,090, and Japanese Laid Open Patent Applications Nos. Jp-A 2002-522915, and Jp-A-Heisei 9-204770.
On the other hand, the yoke, which effectively reduces the write current, may be a potential source of an undesirable bias magnetic field due to the shape anisotropy. The bias magnetic field designates the magnetic field applied to the magnetoresistance element with no write current developed.
FIGS. 35A and 35B illustrate a typical structure of an MRAM. As shown in FIG. 35A, wirings 101, through which write currents flow, are disposed to extend in the x-axis direction, and wirings 102 are disposed to extend in the y-axis. As shown in FIG. 35B, yokes 104 are formed to cover top and side surfaces of the wirings 101, with the ends thereof aligned to the ends of the wirings 101.
The yokes 104 are formed to be shaped correspondingly to the shapes of the wirings 101, to therefore extend along the direction in which the wirings 101 are extended. Such shape anisotropy of the yokes 102 helps the magnetizations of the yokes 104 to be directed along the direction in which the wirings 101 are extended. The reduction in the widths of the wirings 101 enhances the shape anisotropy of the yokes 104, and helps the magnetizations of the yokes 104 to be directed along the direction in which the wirings 101 are extended.
Directing the magnetizations of the yokes 104 along the direction in which the wirings 101 are extended results in that magnetic poles are developed on the ends 104a of the yokes 104 in the x-axis direction. These magnetic poles develop bias magnetic fields along the direction in which the wirings 101 are extended. Let us consider the case where a wiring 101 has a length of 100 μm, a width of 1 μm, and a thickness of 0.3 μm, and a yoke 104 formed of NiFe has a thickness of 50 nm. In this case, the bias magnetic field developed by the magnetic poles on the ends 104 of the yoke 104, and emitted from the ends 104, has an intensity of 10 (Oe) in the x-axis direction at a position where the distance from the end of the wiring 101 along the x-axis is 10 μm, and the distance from the bottom surface of the wiring 101 is 0.1 μm.
The bias magnetic field developed by the yokes exerts various influences on the operations of the MRAMs. Firstly, the bias magnetic field causes variations of the characteristics of the magnetoresistance elements, because the intensity of the bias magnetic field developed by the yokes depends on the positions within the memory cell array. This undesirably reduces the margin of the write currents.
Additionally, the bias magnetic field developed by the yokes reduces the coercive field of the free ferromagnetic layers within the magnetoresistance elements, and thereby enhances the magnetization flipping caused by thermal disturbance, when the direction of the magnetic anisotropy of the magnetoresistance elements are directed orthogonally to the direction of the bias magnetic field developed by the yokes. The generation of the bias magnetic field by the yokes is undesirable, because it deteriorates the data retention reliability of the MRAM.
Japanese Laid Open Patent Application Nos. Jp-A 2002-299574, JP-A 2002-280526, and JP-A 2001-273759 disclose MRAM structures for avoiding magnetic crosstalk; however, the disclosed structures does not addresses dealing with the generation of the bias magnetic field by the yokes.